Hi, I'm ex RN and I made the decision to leave to ensure the services didn't stop me starting on the railway noting the 12 month notice period. It's obviously a bit of a gamble and for me it hasn't lined up exactly. I'm in 2 talent pools and all being well I should start training this year (Conditional offer given 4 months out) so I'll have been outside for just over a year. Kept busy with work here and there but depends on your life and family circumstances if that situation is tolerable.
I've seen people get a few weeks notice for a training course on the railway (source is this forum) to getting 4-6 months. Never seen longer.
Of course every branch of the all three services will offer different levels of flexibility, however, I'm pretty sure that a TOC won't offer you a course 12 months in advance. With terminal leave, annual leave and resettlement then 9 months could be ok but the problem still exists.
For reference, I put my 12 month notice in once I got into a talent pool assuming it would take some time and it has.
If you're offered a course in your notice period then you might have to turn it down but they shouldn't bin you - you just go back into the hold but definitely grab an offer if/when you get one.
Others will have their own experience of transition from the armed forces to the railway so they might pitch in.
Let me know if I can help further.